How can I change Blender's theme?
What you want to change is the theme.
Here is how its done. Open your user preferences Ctrl+Alt+U.1 Then one the top you will see a couple of tabs one of the labelled Themes Then one the left from the drop down you can select a different theme. If you want a darker theme try Elysium. 







And this is what it looks like.



If you want more themes you can get them here:
https://www.blendswap.com/blends/category/22

1: CMD+, on Mac. Edit menu -> Preferences in newer versions.